First, open Finder. If you do not see your hard drive in Finder's sidebar, go to Finder > Preferences > Sidebar, and under "DEVICES," check the box for hard disk. Once completed, click "Macintosh HD" from the sidebar back in Finder, then click System. Next, in the search field in the upper right, enter "mackeeper." Be sure to select "System" for the location that Finder is checking, as shown below:

You probably also want to select your hard drive again, and this time select "Library," and perform the same search.